The present invention relates to a base station, and particularly to a base station from which wireless data of VoIP are transmitted to a terminal.
As a voice calling system using data communications, Voice over IP (VoIP) is given. The VoIP is a technique by which voice as an analog signal is converted into a digital signal to be IP packetized, so as to make a call using an IP network such as the existing Internet. In making a call using the VoIP, it is important to secure the quality of voice and to reduce a delay time. To secure the quality of voice and to reduce a delay time are also important in providing VoIP services using a wireless data communication system.
In a wireless data communication system, a wireless base station transmits data while changing a communication data rate in accordance with conditions of wireless environment where a user terminal utilizing data communication services is located. In general, the data is transmitted at a high data rate to a user terminal which is located in a good wireless environment. On the contrary, the data is transmitted at a low data rate to a user terminal which is located in a bad wireless environment. By appropriately changing the communication data rate, the data can be transmitted to the user terminal with high probability.
As described above, the communication data rate between the base station and the terminal differs in the wireless data communication system depending on the conditions of wireless environment where the terminal is located. In this case, the number of data slots necessary for transmitting one packet differs in accordance with the communication data rate. In general, in the case where data communications are performed at a low data rate, the number of necessary data slots is increased.
With reference to FIGS. 1, 2A and 2B, there will be described a relation between the data rate and the number of necessary slots. Here, FIG. 1 is a diagram for explaining a configuration of a voice packet and the number of slots. Further, FIGS. 2A and 2B are diagrams, each explaining slot assignment.
In FIG. 1, a voice packet 1 is modulated with a modulation signal, and is assigned to slots 1-1 to 1-n. When the data rate of the modulation signal is 153.6 kbits/s, n is 4. Further, when the data rate of the modulation signal is 38.4 kbits/s that is one-fourth of 153.6 kbits/s, n is 16 that is four times of 4. Specifically, when the communication data rate is 153.6 kbits/s, the packet is assigned to 4 slots. When the communication data rate is 38.4 kbits/s, the packet is assigned to 16 slots.
FIG. 2A shows slot assignment in the case where the communication data rate is 153.6 kbits/s. In addition, FIG. 2B shows slot assignment in the case where the communication data rate is 38.4 kbits/s. In FIGS. 2A and 2B, one slot has 1.67 ms of time in a 1×Evolution Data Only (1×EV-DO) system, and the slots are transmitted with an interval of three slots from the time one slot is transmitted to the time the next slot of the same voice packet is transmitted. Thus, it takes 6.68 ms (1.67 ms×four slots). Therefore, in the case of a low data rate such as 38.4 kbits/s in FIG. 2B, it takes up to 106.88 ms (=1.67 ms×4×16) to transmit voice data of one packet, so that delay occurs.
As described above, when the number of data slots to transmit one packet is increased, delay occurs. In the case where delay is increased at the time of using VoIP, there is a problem such as difficulty of hearing voice. Further, an increase in the number of slots used decreases the number of available users.